Method and windowing unit to reduce leakage, fourier...

Details Method and windowing unit to reduce leakage, fourier... Method and windowing unit to reduce leakage, fourier...

1998-07-10

2001-05-29

Pham, Chi (Department: 2631)

Pulse or digital communications

Transceivers

Modems

C375S346000

Reexamination Certificate

active

06240129

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a method to reduce leakage in a fourier transformer, a windowing unit adapted to reduce leakage in a fourier transformer, a fourier transformer and a discrete multi tone modem, both including such a windowing unit and a discrete fourier transform processor coupled to the windowing unit.
2. Discussion of Related Art
Such a method and equipment to perform such a method are already known in the art, e.g. from the international patent application published under the patent cooperation treaty (PCT) with reference number WO 94/21061 and with title ‘A multi-channel digital transmitter and receiver’. Therein, information signals are transmitted from a transmitter to a receiver, digitized in the latter one, windowed and discrete fourier transformed. As is stated on page 11, lines 14-19, spectral leakage may be reduced by increasing the window width using window functions with low sidelobes. As will be explained later on, spectral leakage causes band-limited noise to disturb all frequency components. Consequently, band-limited noise immunity may be improved by reducing spectral leakage using a window filter whose length is increased and whose head and tail are smoothed. The known solution of increasing the window width nevertheless requires an increased number of discrete fourier transform (DFT) points, and thus renders the transformation more complex.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and equipment of the above known type, but which does not involve an increase of the amount of DFT points.
In accord with a first aspect of the invention, a method to reduce leakage in a fourier transformer (FT), comprises the steps of:
a. receiving an incoming signal (X) comprising data symbols with a data symbol length B; and
b. windowing said incoming signal (X) by multiplying it with a window function (W) to thereby generate a sequence of windowed data symbols (X
SW
), said window function (W) comprising a window head (HEAD) with a head length H, a window body (BODY) with a body length which equals said data symbol length B, and a window tail (TAIL) with a tail length T, said windowed data symbols (X
SW
) having a length H+B+T.
is characterized in that said method further comprises the step of:
c. adding a symbol head of said windowed data symbols (X
SW
) to a tail of a symbol body of said windowed data symbols (X
SW
), and similarly adding a symbol tail of said windowed data symbols (X
SW
) to a head of said symbol body of said windowed data symbols (X
SW
) to thereby generate a sequence of folded windowed data symbols (X
SWF
) whose length equals said data symbol length B.
According to a second aspect of the invention, a windowing unit (WFU) for reducing leakage in a fourier transformer (FT), said windowing unit (WFU) including:
a. an input terminal adapted to receive an incoming signal (X) comprising data symbols with a data symbol length B; and
b. a window filter (WI), coupled to said input terminal, and adapted to window said incoming signal (X) by multiplying it with a window function (W) comprising a window head (HEAD) with head length H, a window body (BODY) whose length equals said data symbol length B, and a window tail (TAIL) with tail length T, to thereby generate a sequence of windowed data symbols (X
SW
) whose length equals H+B+T.
is characterized in that said windowing unit (WFU) further includes:
c. a folding processor (F), coupled to said window filter (WI), for adding a symbol head of said windowed data symbols (X
SW
) to a tail of a symbol body of said windowed data symbols (X
SW
), and similarly to add a symbol tail of said windowed data symbols (X
SW
) to a head of said symbol body of said windowed data symbols (X
SW
), to thereby constitute a sequence of folded windowed data symbols (X
SWF
) whose length equals said data symbol length B.
According to a third aspect of the invention, a fourier transformer (FT) comprising a windowing unit (WFU) and a discrete fourier transform processor (DFT) coupled to said windowing unit (WFU), said windowing unit (WFU) including:
a. an input terminal adapted to receive an incoming signal (X);
b. a sampling device (S) adapted to sample said incoming signal (X) to thereby generate a sample sequence (X
S
) comprising data symbols with a data symbol length of B samples, B being an integer value; and
c. a digital window filter (WI), coupled to said sampling device (S), and adapted to window said sample sequence (X
S
) by multiplying it with a window function (W) comprising a window head (HEAD) with head length H, a window body (BODY) whose length equals said data symbol length B, and a window tail (TAIL) with tail length T, to thereby generate a sequence of windowed data symbols (X
SW
) whose length equals H+B+T; and
d. an output terminal coupled to said discrete fourier transform processor (DFT),
is characterized in that said windowing unit (WFU) further includes:
e. a folding processor (F), coupled to said digital window filter (WI), for adding a symbol head of said windowed data symbols (X
SW
) to a tail of a symbol body of said windowed data symbols (X
SW
), and similarly to add a symbol tail of said windowed data symbols (X
SW
) to a head of said symbol body of said windowed data symbols (X
SW
), to thereby constitute a sequence of folded windowed data symbols (X
SW
) with length B and to apply said sequence of folded windowed data symbols (X
SWF
) to said output terminal.
According to a fourth aspect of the invention, a discrete multi tone (DMT) modem (MODEM) comprising between a line terminal (LT) and a modem output (O) a receiving part (R), and between a modem input (I) and said line terminal (LT) a transmitting part (T), said transmitting part (T) including the cascade connection of a serial to parallel converter (SP), a mapper (MAP), an inverse fourier transformer (IFT), a cyclic prefix adder (CPA), a parallel to serial converter (PS) and a digital to analog converter (DAC), said serial to parallel converter (SP) being adapted to parallelize a sequence of transmit bits (TB), said mapper (MAP) being adapted to allocate said transmit bits (TB) to a set of carriers to thereby constitute discrete multi tone (DMT) symbols, said inverse fourier transformer (IFT) being adapted to transform said discrete multi tone (DMT) symbols to time domain, said cyclic prefix adder (CPA) being adapted to add a cyclic prefix to said discrete multi tone (DMT) symbols to thereby protect said discrete multi tone (DMT) symbols for intersymbol interference, said parallel to serial converter (PS) being adapted to serialize said discrete multi tone (DMT) symbols to thereby generate a sequence of discrete multi tone (DMT) symbols, and said digital to analog converter (DAC) being adapted to convert said sequence of DMT symbols to analog signals, and said receiving part (R) including the cascade connection of an analog to digital converter (ADC), a time domain equalizer (TEQ), a serial to parallel converter (SP′), a fourier transformer (FT′), a frequency domain equalizer (FEQ), a demapper (DEMAP), and a parallel to serial converter (PS′), said analog to digital converter (ADC) being adapted to digitize received signals to thereby generate a sequence of received discrete multi tone (DMT) symbols with a discrete multi tone (DMT) symbol length B, said time domain equalizer (TEQ) being adapted to digitally filter said received discrete multi tone (DMT) symbols to thereby compensate for intersymbol interference due to transmission over a transmission line (TL), said serial to parallel converter (SP′) being adapted to parallelize said received discrete multi tone (DMT) symbols, said fourier transformer (FT′) being adapted to transform said received DMT symbols to frequency domain, said frequency domain equalizer (FEQ) being adapted to digitally filter said received discrete multi tone (DMT) symbols to thereby compensate for phase and amplitude distortion due to transmission over said transm

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